Practical_Antenna_Handbook_0071639586
C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 21 Height in kilometers 350 Atomic particles Particle radiation Cosmic rays X-rays Barometric pressure Height in miles 200 300 Nighttime short-wave reflection F2 region 250 200 150 100 50 25 Sea level Temperature –40C –55C –80C +10C Meteor trail Ultraviolet light Sial High-frequency nighttime reflection High-frequency radio waves (1,500-30,000 kHz) Troposphere Stratosphere Medium-frequency radio waves (500-1,500 kHz) Daytime short-wave reflection F1 region E2 region Broadcast-wave reflection E1 region Long-wave reflection D region Ozone layer Low-frequency radio waves (20-500 kHz) Ocean Basaltic layer Isothermal layer 0.000001 mm 0.001 mm 2 mm 760 mm 150 100 50 20 Sea level Figure 2.7 Radio waves in the atmosphere and some external radiation sources.
22 p a r t I I : F u n d a m e n t a l s electrons away from the gas molecules of the ionosphere. These freed electrons are negative ions, while the O 2 and N molecules that lost the electrons become positive ions. The density of the air is quite low at those altitudes, so each free electron can travel a long distance before bumping into a positive ion, at which point they neutralize each other’s electrical charge by recombining. Ionization does not occur at lower altitudes—i.e., in the troposphere and stratosphere—partly because much of the incoming radiation is blocked before it can reach that far and partly because the air density is so much greater at lower altitudes that the positive and negative ions are more numerous and closer together, and recombination occurs rapidly. Because a large percentage of the total radiation “raining” on our atmosphere comes from the sun, ionization levels in the ionosphere vary with the time of day, with the season (since the earth is farther from the sun during the northern hemisphere’s summer), and with the solar radiation levels, both short term and long term. Ionization of the upper level of the earth’s atmosphere by radiation from space causes the ionosphere to have electrical characteristics not shared by the lower levels. While the details are beyond the scope of this book, the net effect is to set up the possibility of electrical interaction between the ionosphere and radio waves that reach it. Not surprisingly, the effects of the interactions are frequency dependent. A second effect of radiation from space is to alter the characteristics of bands of magnetism (so-called magnetic belts) encircling our globe. When these bands are disturbed by excessively high cosmic radiation, they alter the magnetic fields surrounding the earth, causing disruption of normal EM propagation. As with ionization effects, some frequencies are affected more than others. As we shall see, the ionization of our upper atmosphere is a major factor in the extreme variability of medium- and high-frequency radio-wave propagation. It is, quite simply, the “stuff” of magic for those of us who have been mesmerized by the unpredictability of long-distance terrestrial radio communications on those bands. EM Wave Propagation Phenomena If you have ever studied optics, you know that the path and polarization of visible light can be modified through reflection, refraction, diffraction, and dispersion. Radio waves (which, like visible light, are electromagnetic waves) can be affected the same way. Figures 2.8A and 2.8B illustrate some of the wave behavior phenomena associated with both light and radio waves. All four effects listed here play important roles in radio propagation. Reflection and refraction are shown in Fig. 2.8A. Reflection occurs when a wave strikes a denser reflective medium, such as when a light wave in air strikes a glass mirror. The incident wave (shown as a single ray) strikes the interface between less dense and more dense media at a certain angle of incidence (a i ), and is reflected at exactly the same angle, called the angle of reflection (a r ). Because these angles are equal, a light beam or radio signal undergoing pure reflection can often be traced back to its origin. If the incoming light wave arrives at certain other angles of incidence at the boundary between media of two different densities, refraction is the result. The amount and direction of the change are determined by the ratio of the densities between the two media. If Zone B is much different from Zone A, the bending is pronounced. In radio systems, the two media might be different layers of air with different densities. It is pos-
- Page 2 and 3: Practical Antenna Handbook
- Page 4 and 5: Practical Antenna Handbook Joseph J
- Page 6 and 7: Contents Preface . . . . . . . . .
- Page 8 and 9: C o n t e n t s vii 12 The Yagi-Uda
- Page 10 and 11: C o n t e n t s ix Switched-Pattern
- Page 12 and 13: Preface My paternal grandfather was
- Page 14 and 15: P r e f a c e xiii this book repres
- Page 16 and 17: Acknowledgments As with other field
- Page 18 and 19: Background and History Part I Chapt
- Page 20 and 21: CHAPTER 1 Introduction to Radio Com
- Page 22 and 23: C h a p t e r 1 : I n t r o d u c t
- Page 24 and 25: Fundamentals Part II Chapter 2 Radi
- Page 26 and 27: CHAPTER 2 Radio-Wave Propagation To
- Page 28 and 29: C h a p t e r 2 : r a d i o - W a v
- Page 30 and 31: C h a p t e r 2 : r a d i o - W a v
- Page 32 and 33: C h a p t e r 2 : r a d i o - W a v
- Page 34 and 35: C h a p t e r 2 : r a d i o - W a v
- Page 36 and 37: C h a p t e r 2 : r a d i o - W a v
- Page 40 and 41: C h a p t e r 2 : r a d i o - W a v
- Page 42 and 43: R N-1 C h a p t e r 2 : r a d i o -
- Page 44 and 45: C h a p t e r 2 : r a d i o - W a v
- Page 46 and 47: C h a p t e r 2 : r a d i o - W a v
- Page 48 and 49: C h a p t e r 2 : r a d i o - W a v
- Page 50 and 51: C h a p t e r 2 : r a d i o - W a v
- Page 52 and 53: C h a p t e r 2 : r a d i o - W a v
- Page 54 and 55: C h a p t e r 2 : r a d i o - W a v
- Page 56 and 57: C h a p t e r 2 : r a d i o - W a v
- Page 58 and 59: C h a p t e r 2 : r a d i o - W a v
- Page 60 and 61: C h a p t e r 2 : r a d i o - W a v
- Page 62 and 63: C h a p t e r 2 : r a d i o - W a v
- Page 64 and 65: C h a p t e r 2 : r a d i o - W a v
- Page 66 and 67: C h a p t e r 2 : r a d i o - W a v
- Page 68 and 69: Figure 2.29C Monthly averaged sunsp
- Page 70 and 71: C h a p t e r 2 : r a d i o - W a v
- Page 72 and 73: C h a p t e r 2 : r a d i o - W a v
- Page 74 and 75: C h a p t e r 2 : r a d i o - W a v
- Page 76 and 77: C h a p t e r 2 : r a d i o - W a v
- Page 78 and 79: C h a p t e r 2 : r a d i o - W a v
- Page 80 and 81: C h a p t e r 2 : r a d i o - W a v
- Page 82 and 83: C h a p t e r 2 : r a d i o - W a v
- Page 84 and 85: C h a p t e r 2 : r a d i o - W a v
- Page 86 and 87: C h a p t e r 2 : r a d i o - W a v
C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 21<br />
Height<br />
in kilometers<br />
350<br />
Atomic particles<br />
Particle radiation<br />
Cosmic rays<br />
X-rays<br />
Barometric<br />
pressure<br />
Height<br />
in<br />
miles<br />
200<br />
300<br />
Nighttime short-wave<br />
reflection F2 region<br />
250<br />
200<br />
150<br />
100<br />
50<br />
25<br />
Sea<br />
level<br />
Temperature<br />
–40C<br />
–55C<br />
–80C<br />
+10C<br />
Meteor<br />
trail<br />
Ultraviolet light<br />
Sial<br />
High-frequency nighttime reflection<br />
High-frequency radio waves (1,500-30,000 kHz)<br />
Troposphere<br />
Stratosphere<br />
Medium-frequency radio waves (500-1,500 kHz)<br />
Daytime short-wave<br />
reflection F1 region<br />
E2 region<br />
Broadcast-wave<br />
reflection E1 region<br />
Long-wave<br />
reflection<br />
D region<br />
Ozone layer<br />
Low-frequency radio waves (20-500 kHz)<br />
Ocean<br />
Basaltic layer<br />
Isothermal layer<br />
0.000001 mm<br />
0.001 mm<br />
2 mm<br />
760 mm<br />
150<br />
100<br />
50<br />
20<br />
Sea<br />
level<br />
Figure 2.7 Radio waves in the atmosphere and some external radiation sources.
Change language